A torque sensor has been known that detects the torque generated on an input shaft and an output shaft coupled together via a torsion bar as an amount of torsion (relative rotational position) between the input shaft and the output shaft. For example, Patent Literature 1 mentioned below discloses a torque sensor in which aluminum cylindrical body having a plurality of opening windows forming two rows in a circumferential direction are attached to each of input and output shafts, both the cylindrical bodies are placed such that the two opening window rows of those overlap with each other, detection coils are placed to the respective opening window rows. The change in the overlap of the opening windows in each of the rows according to the amount of torsion (relative rotational position) is detected by the detection coils. However, since the torque sensor disclosed in Patent Literature 1 does not have sufficient compensation for the temperature drift characteristic of the sensor components, the torque sensor has difficulty in obtaining high accuracy of detection and in obtaining a wider dynamic range of the detection output signals and further does not have sufficient failure management for the sensor components.
Patent Literature 2 mentioned below discloses that a torque sensor is provided with a temperature-compensating resistor (such as a thermistor) that is connected with a detection coil in series to compensate the temperature drift characteristic of the detection coil. Further, in Patent Literature 2, the phases of an output AC voltage of the detection coil and excitation AC voltage are synchronized, and then the difference between those voltages are obtained by use of a differential amplifier circuit. The differential voltage is sampled and held at a peak position, thereby obtaining the analogue DC voltage corresponding to a detected torque. However, even such a configuration results in insufficient and limited dynamic range of the detection output signal. For example, an attempt to obtain a seemingly large dynamic range by increasing the amplification factor of the differential amplifier circuit will result in an inappropriate signal-to-noise ratio. A sampling pulse generating circuit for generating a sampling pulse for a sample-and-hold control is configured to perform analogue processing of excitation of AC voltage. Accordingly, this analogue circuit section has a temperature drift characteristic, and it may adversely influence on the detection accuracy.
On the other hand, Patent Literatures 3 to 5 mentioned below disclose torque sensors including failure diagnostic function. In those conventional technologies, failures such as disconnection and partial disconnection, abnormality, and so forth are detected by use of either the phase or the DC component of the output AC signal. However, a toque sensor is still desired that includes a general failure diagnostic function in consideration of both the phase and the DC component of the output AC signal in a simple configuration.
However, in the conventional technologies, each type of failure diagnostic function belongs to an individual diagnostic circuitry. Accordingly, in an extreme case, a diagnosis result has to be transmitted to a central control unit (for example, an ECU in an automobile) via a separate line for each type of failure detection signal, thus resulting in an increased number of lines. For example, in a failure diagnosis in consideration of the DC component of the output AC signal from the detection coil in the conventional technology, the output AC signal is filtered by a low-pass filter, thereby extracting its DC component and evaluating the extracted DC voltage component. Therefore, as an output line from a sensor section to the central control unit, a particular line for the DC voltage component output from the low-pass filter has to be provided separately from an output AC signal line. This causes a problem of increased transmission lines.
Patent Literature 1: JP-A-Hei 8-114518
Patent Literature 2: JP-B-3588684
Patent Literature 3: JP-B-3589053
Patent Literature 4: JP-B-3649057
Patent Literature 5: JP-B-4043116